Ceramic metal composite for orthopaedic implants

ABSTRACT

The invention relates to an orthopedic implant made of a ceramic metal composite. The composite ( 28, 48, 54 ) includes one phase that is a biocompatible metal or metal alloy and a second phase of ceramic particles examples of which include carbides, nitrides and/or oxides. In some embodiments, the implant comprises a homogeneous ceramic layer ( 24 ) as part of a multi-layered composition. In some embodiments, the multilayered composition comprises a homogeneous metal layer ( 32 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationSer. No. 60/790,918 filed Apr. 11, 2006, which is incorporated byreference as though fully described herein.

TECHNICAL FIELD

The present invention relates to ceramic/metal composite materials fororthopedic implants generally, and more specifically, to such materialsfor hip implants.

BACKGROUND OF THE INVENTION

For hip implants, the current standards of “hard-on-hard” bearings areceramic-on-ceramic and metal-on-metal. The ceramics are inherentlybrittle in nature and are always associated with a finite risk offracture. There are also limitations on the size of the ceramiccomponents that can be made. Large size ceramic components (especiallyliners) have to be thin and may have higher risk of fracture. Themetal-on-metal components have only insignificant fracture risk andlarger size components can be made. The current standard material ofmetal-on-metal implants is high carbon Co—Cr (cobalt-chromium) alloy.The major concern with the metal-on-metal implant is the metal ionrelease from the joint and its unknown effects on the physiology of thehuman body. The advantage of metal-on-metal implants is that they can beused in larger sizes. The larger size of the implant allows greaterrange of motion. The metal-on-metal implants have also been shown to beuseful for resurfacing types of applications where conservation of boneis desired. In such larger joints, conventional polyethylene orcross-linked polyethylene are not preferred as a counter-bearing surfaceand metal-on-metal is typically the only other alternative. This is dueto the fact that the larger size requires a polyethylene liner to bethinner. A thinner liner may have less mechanical strength, may haveincreased creep, and may lead to increased wear and osteolysis andeventually to the failure of the implant.

The other commonly used hard-on-hard implant material isceramic-on-ceramic. The current standard material of ceramic-on-ceramicimplants is alumina. Metal ion release is typically not a concern forthese implants. But due to limited toughness and the brittle nature ofceramics, it is difficult to make these implants in larger sizes. Theceramic components have finite probability of fracture thus leading to apotential joint failure and complications associated with the fractureof a joint.

It has been an object of much of the prior art to reduce the metal ionrelease and minimize the fracture risk by combining metal and ceramiccomponents. One of the prior art approaches to reduce the risk of metalion release is to use surface hardening of the head or liner or bothusing diffusion or plasma processes to incorporate nitrogen and/orcarbon on the surface of the alloy. Another approach is to coat themetallic surface with ceramic coatings of nitrides (titanium nitride,chromium nitride, etc.), oxides (aluminum oxide, zirconium oxide,zirconium-alumina oxide, etc.) or diamond like carbon or diamondcoatings. Another approach is to use a metal head on a ceramic liner orvice versa. In this approach, fracture risk is reduced along with themetal ion release. Another approach that has been used is the reductionof the diametrical clearance between the articulating components therebyforming a thick lubricating film which assists in the reduction of wear.Fisher et al (U.S. Patent Publication No. 2005/0033442) and Khandkar etal. (U.S. Pat. No. 6,881,229) teach the use of a metal-on-ceramicarticulation. Fisher et al teach that the difference in hardness betweenthe metallic component and the ceramic component should be at least 4000MPa. Khandkar et. al. specifically teach use of silicon nitride ceramiccomponents for articulation against the metallic component. In bothinstances, the objective is to lower the wear of mating couples. But inboth instances, the fracture risk of ceramic is still significant.

Ceramic-metal composites have also seen application in the prior art.U.S. Pat. No. 6,620,523 discloses ice skating blades made from a metalmatrix composite. The ice skating blade has a titanium core and a metalcomposite material cladding. The metal composite material may becomprised of titanium or zirconium. U.S. Patent Application PublicationNo. 2004/0243241 discloses an orthopedic device, such as a spinalimplant, formed of a metal matrix composite. The metal matrix compositeincludes a biocompatible metal alloy and a reinforcing component, suchas a hard or refractory material.

However, there remains a need in the art for improved articulationjoints that reduce the risk of metal ion release and the risk ofmonolithic ceramic fracture. There further remains a need in the art forimproved articulating joints wherein one or both of the articulatingsurfaces are comprised of a metal matrix composite.

One particular advantage of the present invention is the reduction ofthe risks of fracture and metal ion release for orthopedic implants. Therisks are reduced by using a graded ceramic metal composite componentwith ceramic surface and graded surface below the ceramic surface. Asmentioned in the details of the invention, the graded ceramic metalcomposite of present invention provides a solution to the abovedescribed problems pertaining to currently used hard-on-hard bearings.Although the present invention is particularly applicable to hipimplants, it is also useful for orthopedic implants generally. Asexamples of other embodiments of the invention, the compositiondescribed herein is applicable to knee and spinal implants and otherimplants wherein hard-on-hard articulation is desired.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to ceramic/metal composite materialsfor orthopedic implants generally, and more specifically, to suchmaterials for hip implants.

In one aspect of the present invention, an orthopedic implant componentcomprises: a homogenous composition comprising a metallic species and asecondary phase wherein said secondary phase comprises a ceramic andwherein the average free distance between ceramic particles is less thanthe average size of said ceramic particles; or, a multilayeredcomposition wherein one or more layers comprise a ceramic, a metallicspecies, or a combination of a ceramic and a metallic species.

In some embodiments, the ceramic is selected from the group consistingof alumina, zirconia, chromium carbide, chromium nitride, siliconcarbide, silicon nitride, titanium carbide, zirconium carbide, zirconiumnitride, tantalum carbide, tungsten carbide, and any combinationthereof.

In some embodiments, the metallic species is selected from the alloygroups consisting of cobalt-chromium, titanium-aluminum-vanadium,zirconium-niobium and tantalum and any combination thereof.

In some embodiments, wherein said multilayered composition has a surfacelayer of homogeneous ceramic, and a subsurface layer comprising ametallic species and a secondary phase comprising a ceramic immediatelybelow said surface layer and forming a boundary with said surface layer.

In some embodiments, the surface layer of homogeneous ceramic has athickness of about 25 μm.

In some embodiments, the surface layer is 100% alumina.

In some embodiments, the surface layer is 80% alumina by volume and 20%zirconia by volume.

In some embodiments, the subsurface layer comprising a metallic speciesand a secondary phase comprising a ceramic is a graded layer wherein thelevel of ceramic decreases from an initial concentration at the boundarywith said surface layer to depths further from said boundary with saidsurface layer.

In some embodiments, the implant component further comprises ahomogenous metallic substrate layer immediately below said graded layer.

In some embodiments, the metallic substrate layer is a homogeneouscobalt-chromium alloy.

In some embodiments, the concentration at said boundary of said ceramicin said subsurface layer is 98% by volume.

In some embodiments, the implant component comprises a multilayeredcomposition, wherein one layer comprises a combination of a ceramic anda metallic species and forms a surface of said implant component andwherein said one layer is a graded layer wherein the level of ceramicdecreases from an initial concentration at said surface to depthsfurther from said surface.

In some embodiments, the implant component comprises a multilayeredcomposition, wherein one layer comprises a combination of a ceramic anda metallic species and forms a surface of said implant component andwherein said one layer is a graded layer wherein the level of ceramicdecreases from an initial concentration at said surface to depthsfurther from said surface, the concentration at said surface of saidceramic is 98% by volume.

In some embodiments, the implant component comprises a homogenouscomposition comprising a metallic species and a secondary phase whereinsaid secondary phase comprises a ceramic and wherein said ceramiccomprises alumina.

In some embodiments, the ceramic is a mixture of alumina and zirconia.

In some embodiments, the implant component comprises a hip implant.

In some embodiments, the implant component comprises a knee implant.

In some embodiments, the implant component comprises a homogenouscomposition comprising a metallic species and a secondary phase whereinsaid secondary phase comprises a ceramic, and wherein the average freedistance between ceramic particles is less than half of the average sizeof said ceramic particles.

In some embodiments, the implant component comprises a multilayeredcomposition wherein one or more layers comprise a combination of aceramic and a metallic species and wherein the average free distancebetween ceramic particles is less than the average size of said ceramicparticles.

In some embodiments, the implant component comprises a multilayeredcomposition wherein one or more layers comprise a combination of aceramic and a metallic species and wherein the average free distancebetween ceramic particles is less than half of the average size of saidceramic particles.

In another aspect of the present invention, a method of making anorthopedic implant component having a graded composition comprises thesteps of: mixing powders of ceramic components and metal components todesired ratios to form a first mixture; mixing powders of ceramiccomponents and metal components to desired ratios to form a secondmixture; laying said first mixture on said second mixture to create afirst layered mixture; optionally mixing powders to form one or moreadditional mixtures and laying said one or more additional mixtures onsaid first layered mixture; and, sintering said first layered mixture orsintering a combination of said first layered mixture and said one ormore additional mixtures.

In some embodiments, at least one mixing step comprises mixing using aball milling process.

In some embodiments, at least one mixing step comprises conventionalhigh temperature sintering.

In some embodiments, at least one mixing step comprises electric fieldassisted sintering.

In some embodiments, at least one mixing step comprises spark plasmasintering.

In some embodiments, the metal components have a Young's modulus in therange of about 60 GPa to about 150 GPa.

In some embodiments, the method further comprises the step of alteringthe size and volume fraction of ceramic particles in a ceramic/metalmixture such that the average free distance between ceramic particles isless than the size of said ceramic particles.

In some embodiments of the method, the step of altering the size andvolume fraction of ceramic particles in a ceramic/metal mixture suchthat the average free distance between ceramic particles is less thanthe size of said ceramic particles comprises altering the size andvolume fraction such that the average free distance between ceramicparticles is less than half of the size of said ceramic particles.

Further features, aspects, and advantages of the present invention, aswell as the structure and operation of various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims. The novel features which are believed to be characteristic ofthe invention, both as to its organization and method of operation,together with further objects and advantages will be better understoodfrom the following description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a hip prosthesis which is one embodiment of thepresent invention;

FIG. 2 shows a cross-sectional view of an example of an embodiment of amultilayered structure of the present invention;

FIG. 3 shows a cross-sectional view of an example of an embodiment of amultilayered structure of the present invention;

FIG. 4 shows a cross-sectional view of an example of an embodimenthaving a homogenous composition comprising a metallic species and asecondary phase wherein the secondary phase comprises a ceramic;

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “a” or “an” means one or more, unless otherwiseindicated. The singular encompasses the plural and the pluralencompasses the singular. Thus, where reference is made to “a ceramic”or to a “ceramic component”, this means one or more ceramics and one ormore ceramic components, respectively. Where reference is made to “ametal” or to a “metallic species”, this means one or more metals and oneor more metallic species, respectively. A metallic species may be onemetal or a metal alloy.

As used herein the term “homogeneous ceramic” encompasses, but is notlimited to, 100% of one specific ceramic (for example, 100% alumina). Inthis way, the term “homogenous ceramic” also encompasses a mixture ofceramics (for example a mixture of zirconia and alumina and possiblyother ceramics) so long as it is substantially free of non-ceramiccomponents. Similarly, the term “homogeneous metal” or “homogenousmetallic component” covers, but is not limited to, 100% of one specificmetal (for example, 100% zirconium). In this way, the term “homogenousmetal” or “homogenous metallic component” also encompasses a mixture ofmetals (for example a mixture of zirconium and hafnium and possiblyother metals) so long as no non-metal is present.

As used herein, the term “orthopedic implant component” is defined as aportion of an entire orthopedic implant or, an entire orthopedicimplant.

Wherein compositions are recited in percentages, the percentages arevolume percentages, unless otherwise indicated.

The invention relates to an orthopedic implant made of a ceramic metalcomposite. The composite includes a first phase that is a biocompatiblealloy and a second phase that is ceramic particles of carbides,nitrides, borides and/or oxides. The ceramic layer can be present on thearticulating surface from about 1 % to about 100% by volume (preferablyfrom about 1% to about 100% by volume) in the metal. The volume fractionof the ceramic phase may be graded from the articulating surface of theimplant to the non articulating substrate. The composite may be furtherhardened by allowing diffusion of nitrogen, oxygen, carbon orcombination thereof. The ceramic composite may be further coated withhard coatings of carbides, nitrides, oxides, or diamond-like carbon orpolycrystalline-nanocrystalline diamond coatings or amorphous diamondcoatings.

The approach of the present invention is to use a ceramic metalcomposite where the ceramic layer is part of the metal matrix. Theceramic layer can be present from about 1% to about 100% by volume inthe metal. This layer provides surface hardening to the metal, reducesmetal ion release because the ceramic particles articulate against eachother instead of metallic particles articulating against each other, andalso mitigates the fracture risk associated with monolithic ceramics.The primary goal is to reduce metal ion release in metal-on-metal typearticulation but the hardened surface can also be used for articulationagainst other materials.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. Most of the following discussion focuses onhip implants for illustrative purposes, but it should be clear that thepresent invention is also applicable to other orthopedic implants.

FIG. 1 illustrates a hip prosthesis 10. The hip prosthesis 10 includes astem 12, a femoral head 14, a liner 16, and a shell 18. The femoral head14 is operatively connected to the stem 12, and the liner 16 isassembled to the shell 18. The stem 12 is adapted for mounting in afemur (not shown), and the shell 18 is adapted for mounting to anacetabulum (not shown). The femoral head 14 articulates against theliner 16.

In some embodiments of the present invention, the femoral head 14 and/orthe liner 16 are made of a ceramic metal composite where the ceramiclayer is part of the metal matrix. In one embodiment, a surface layer of100% ceramic of thickness of ca. 5 to 200 μm is followed by a gradedlayer of ceramic/metal composite having an initial composition of 100%ceramic and decreasing to pure metal or to a steady state level ofceramic. In another embodiment, the surface composition may compriseceramic from about 1% to nearly 100% (the surface layer having a smallamount of metal). The ceramic layer can be present from about 1% toabout 95% by volume in the metal. This layer provides surface hardeningto the metal and the presence of ceramic particles articulating againsteach other reduces the metal ion release and also mitigates the fracturerisk associated with monolithic ceramics. In the hip implant 10embodiment depicted in FIG. 1, both the femoral head 14 and the liner 16of the acetabular component 18 are made from the ceramic metalcomposite, but those skilled in the art would understand that in someinstances only one of the components is made of the ceramic metalcomposite. For example, the ceramic metal composite of the femoral head14 may articulate against a polyethylene liner, monolithic ceramicliner, or a metal liner. Alternatively, a ceramic metal composite liner16 may articulate against a polymeric femoral head (such as one made ofpolyethylene), monolithic ceramic femoral head, or a metal femoral head.As those skilled in the art would understand, the term “polyethylene”includes both conventional polyethylene and cross-linked polyethylene.Also in the hip implant 10 embodiment of FIG. 1, a portion of thefemoral stem 12 is illustrated for proper context. Although FIG. 1illustrates the application of the present invention to hip implants, itshould be understood that this is merely one illustrative example andthat the present invention is applicable to all orthopedic implants,such as, but not limited to, knee implants, shoulder implants, elbowimplants, and other implants.

The composite includes a first phase that is a biocompatible alloy(cobalt chromium based, titanium based, zirconium based, niobium basedand/or tantalum based) and a second phase that is ceramic particles ofcarbides, nitrides, borides and oxides, such as but not restricted tochromium carbide, titanium carbide (TiC), titanium carbo-nitride (TiCN),zirconium carbide (ZrC), tantalum carbide (TaC), niobium carbide (NbC),tungsten carbide, aluminum oxide (Al₂O₃), and zirconium oxide (ZrO₂).The second phase can also include polycrystalline, amorphous and ornano-crystalline diamond grit. The composite may include one type ofceramic phase or a combination of more than two or more types of ceramicphases.

The hardness of the femoral head 14 and the liner 16 may be the same orthe hardness for each component may be different. For example, theceramic composite femoral head 14 may articulate against a monolithicceramic liner 16 in which the hardness of the ceramic particle in thecomposite is different from the monolithic ceramic. As another example,the ceramic composite femoral head 14 may articulate against a ceramiccomposite liner 16 in which the hardness of the ceramic particle in thefemoral head is different from that of the liner. The volume fraction ofthe ceramic phase can be varied from about 1% to about 100% in order foreach of the components to have a different hardness. Alternatively, thecomponents can be made in such a way that the volume fraction of theceramic phase is graded from the surface of the implant to thesubstrate. For example, there may be approximately 100% of the ceramicphase at the surface and gradually decrease the volume fraction of theceramic phase to less than about 1 percent at the core of the substrate.The objective of the graded composite is not to significantly affect thefracture toughness of the substrate material.

In some embodiments of the invention, this graded structure can beachieved using powder metallurgical processes. In such processes thepowders of the ceramic component and the metal component are mixed andsintered. The mixing can be achieved using a ball milling process wherea uniform distribution of the ceramic phase is required. Such processesare known to those of ordinary skill in the art. To obtain the gradedstructure, powders can be applied in the form of layers and thensintered together. The sintering of powders can be achieved using aconventional high temperature sintering process or using lowertemperature electric field assisted sintering. Such sintering methods,known to those of ordinary skill in the art, as well as other sinteringmethods, are useful in the present invention. The temperature of thesintering will depend on the alloy composition and type of structuredesired. Other alloying techniques, such as electron beam alloying ofthe surface, can also be done to achieve such graded structures. In onenon-limiting example of the use of a powder metallurgical process, acomposite structure of alumina and CoCr is made. CoCr powder is mixedwith alumina powder in volume fractions ranging from 25% to 100%. Thesemixtures were then poured in a graphite mould as different layers. Thefirst layer is 100% CoCr. The second layer is CoCr with 25% alumina. Thethird layer is 50% CoCr and 50% alumina. The fourth layer is 75% aluminaand 25% CoCr. The top (surface) layer was 100% alumina. This layeredstructure was then sintered using a spark plasma sintering process at1200° C. Spark plasma sintering is another sintering technique known inthe art. The surface is 100% alumina and fraction of alumina decreasestowards the substrate. The alumina powder had large particle size rangethat resulted in formation of big discontinuities in the matrix. It willbe understood by those skilled in the art that the powder size anddistribution can further be optimized. In such composite structure the100% alumina is expected to be the articulating surface. Such surface isexpected to eliminate the metal ion release. The graded compositionensures that the adhesion of alumina to the substrate CoCr is adequate.In the example described, the change in graded composition may berelatively abrupt. It will be understood by those skilled in the artthat this change in composition can be changed from step function (moreabrupt change) to linear or to any other shape desired. It will beunderstood by those skilled in the art, that sintering of such powdermixtures can be achieved using conventional sintering process. In oneaspect of invention, such structure could also be formed by selectivelymelting the surface and then alloying it with ceramic particles. Inanother aspect, ceramic particles may be precipitated duringsolidification of the molten alloy. In another aspect the ceramicparticles may be dispersed in a molten bath of alloy and then themixtures is cooled to obtain uniform distribution of ceramic particles.

In some embodiments, the ceramic metal composite is formed using lowmodulus alloys, such as but not limited to titanium, zirconium, orniobium. The Young's modulus of these alloys typically range from about60 to about 150 GPa. The lower modulus alloy leads to lower contactpressure and allows for the formation of thicker fluid film lubrication.The thicker fluid film reduces the wear of the articulating components.The low modulus alloy is also selected to help in getting largerclearances between the articulating components, which lowers the chancesof seizure of the joint. In one aspect of invention the diametricalclearance between the mating components of the hip joint is maintainedabove 150 micron.

In some embodiments, the composite is engineered in such as way that theaverage free distance between the ceramic particles is less than theaverage size of the ceramic particles. This is achieved by altering theaverage diameter (size) of the ceramic particles and their volumefraction in the metal. The volume fraction of particles (V_(p)), themean free distance between these particles (λ), and the average size ofthe particles expressed as mean intercept length (L₃) can be related toeach other as provided below [Metals Handbook Ninth Edition, Volume 9,Metallography and Microstructures, ASM, 1989):

$\lambda = {\frac{L_{3}}{V_{p}}\left( {1 - V_{p}} \right)}$

For a known particle size (L₃) and desired distance between theparticles (λ) a suitable volume fraction of particles (V_(p)) can bechosen as a starting point. This mixture then can be further evaluatedanalytically by known and standard techniques to verify the aboveexpressed relationship. In preferred embodiments, the average freedistance (λ) is less than mean intercept length (L₃) by at least 10%.The above process can be repeated iteratively to optimize the meandistance between the particles. The average diameter may be calculatedby measuring the particle diameter of the particles through the centroidof the particles and averaging the measured diameters. For the case ofnon-spherical particles, the diameter may be measured through thecentroid of the particles to different locations on the perimeter of theparticle. For example, for an ellipsoid shaped particle, the diameterwill be determined at different locations of the particle the andaverage of same may be determined. Standard stereological software,familiar to those of skill in the art can be used for such a purpose. Ingeneral particles can range from sub-micron sized (<1 micron) to aslarge as about 200 microns. Based on the particle size opticalmicroscope or scanning electron microscope can be used to measure theparticle sizes. Preferably, at least 10 to 15 fields at of view at 1000×chosen randomly are inspected. It may be required to inspect more fieldsof view if a magnification higher than 1000×is used. It will beunderstood by those skilled in the art that the accuracy and precisionof such measurements can be increased by inspecting a greater number offields. Standard guidelines known to those of ordinary skill in the art(such as, for example, the ASTM guidelines) can be used for thispurpose. By controlling the average free distance between the ceramicparticles, the risk of the harder ceramic articulating against thesofter substrate can be reduced. In such composite, a 100% ceramic isnot always necessary, although it is within the scope of the inventionand may be used. In each of two or more composite componentsarticulating against each other in an orthopedic implant, the followingcan be controlled independently: (1) the type of ceramic particles, (2)the volume fraction of the ceramic particles, (3) the size of theceramic particles, (4) the size distribution of the ceramic particles,(5) the shape of the ceramic particles, and (6) the metallic substratematerial. One or more types of ceramic particles with independentlycontrolled size distributions and volume fractions can be used in thecomposite.

In some embodiments, the composite is engineered in such as way that theaverage free distance between the ceramic particles is less than thesize of the ceramic particles. In some embodiments, the average freedistance between the ceramic phases is less than the size of the ceramicparticles. This is achieved by altering the average diameter (size) ofthe ceramic particles and their volume fraction in the metal. Theaverage diameter may be calculated by averaging the multitudes of thediameter of a particle through the centroid of the particle. Standardstereological software, familiar to those of skill in the art can beused for such purpose. In general particles can range from sub-micronsized (<1 micron) to as large as about 200 microns. By controlling theaverage free distance between the ceramic phases, the risk of the harderceramic articulating against the softer substrate can be reduced. Insuch composite the surface need not be 100% ceramic. In someembodiments, the composite is engineered in such as way that the averagefree distance between the ceramic particles is less than half of thesize of the ceramic particles.

If the surface of such composite is not 100% ceramic then it can befurther hardened by allowing diffusion of nitrogen, oxygen, carbon, orany combination thereof, using plasma or ion implantation type ofprocesses to gain additional advantage of the hardened layer. Thecomposite is further hardened by allowing diffusion of reactive speciessuch as boron, carbon, nitrogen and oxygen to gain additional advantageof the hardened layer in reducing the wear of the component. Thediffusion processes can be accompanied by conventional high temperaturediffusion processes of reactive species or by ion implantation or byplasma assisted process. The diffusion hardened depth of such compositescan range from about 1 micron to about 1000 microns.

In some embodiments, the ceramic composite is further coated with hardcoatings of carbides (TiC, ZrC, TaC etc) or nitrides (TiN, ZrN, CrN etc)or oxides (Al₂O₃, ZrO₂) or borides or diamond like carbon orpolycrystalline/nanocrystalline diamond coatings to reduce the metal ionrelease. These coatings may be applied using physical vapor or chemicalvapor deposition processes. As an example, a coated femoral head mayarticulate against a coated or uncoated liner. Alternatively, a coatedliner may articulate against a coated or uncoated femoral head.

The surface roughness of the femoral head 14 and/or the liner 16 may bespecified such that the finished surface allows for maximum lubricationduring articulation. Moreover, the diametrical clearance between thefemoral head 14 and the liner 16 may be specified such that the finishedsurface allows for maximum lubrication during articulation.

Three illustrative and non-limiting embodiments of the present inventionwill now be described. It should be understood that these do notrepresent the entire scope of the invention. Modifications that will beunderstood by those having ordinary skill in the art are also part ofthe present invention. This includes the use of additional layers ofmaterial, as well as different compositional characteristics of eachlayer.

In a first illustrative non-limiting embodiment of the presentinvention, in FIG. 2 there is an orthopedic implant component 20 (across-sectional portion of which is shown) comprising a surface layer 24of a homogeneous ceramic material. This surface layer 24 could be, forexample, 100% alumina. Alternatively, the surface layer 24 could be amixture of ceramics, for example, 80% alumina and 20% zirconia byvolume. However, other possible homogeneous ceramic materials, known tothose of ordinary skill in the art also can be used. Some examplesinclude mixtures of silicon nitride and silicon carbide, chromiumcarbide and titanium carbide. In general, several such combinations ofceramic particles can be made. The thickness of this ceramic surfacelayer 24 is preferably about 25 μm, however, other thicknesses may beused. The thickness of the ceramic layer will depend on the adhesion ofthe ceramic particles to each other and to the substrate at theinterface. The thickness should be such that it will not de-laminate theceramic layer from the surface. Below the homogeneous ceramic layer,there is ceramic/metal mixture comprising a graded structure 28 of metaland ceramic. In the illustrative example using alumina as a ceramicmaterial in this second layer, the alumina would have an initialconcentration which decreases monotonically as the depth from thesurface increases. Preferably, the initial concentration is 95% to 99%by volume but it may be 100% by volume or some level below 95% by volumeand then monotonically decreases. Below this layer, there is ahomogeneous metal layer 32. In one example, the second layer 28 couldhave an initial concentration (directly below the first layer ofhomogeneous ceramic) of 95% ceramic and 5% metal by volume and decreasein ceramic content to a final concentration of 10% ceramic and 90% metalby volume. The third layer 32, which can be 100% metal by volume thenbegins. The thickness of the individual layers can range from 5 micronsto 500 microns; the ceramic volume fraction can decrease as a stepfunction (abrupt change) or can decrease in a linear fashion. The layerscomprising ceramic/metal mixtures may be characterized by an averagefree distance between ceramic particles that is less than half theaverage size of said ceramic particles. However, this average freedistance between ceramic particles is not required in multilayeredcompositions.

A second illustrative non-limiting embodiment 40 is shown in FIG. 3.This embodiment differs from the first embodiment in that the secondgraded layer 48 decreases to some steady-state value of ceramic/metal.The second graded layer 48 is below a surface layer 24 of a homogeneousceramic composition. In preferred embodiments, the steady-state level isprimarily metallic (for example, 10% ceramic and 90% metal by volume).However, it can be primarily ceramic as well. In this embodiment, thereis no discrete third layer and the final steady-state composition formsthe substrate of the material. Thus, in FIG. 3 we see a structure 40having homogeneous ceramic layer 24 and graded ceramic/metal layer 48.At some depth within layer 48, there is reached a steady-statecomposition of ceramic and metal which then forms the remainder of thesubstrate. The layers comprising ceramic/metal mixtures may becharacterized by an average free distance between ceramic particles thatis less than half the average size of said ceramic particles. However,this average free distance between ceramic particles is not required inmultilayered compositions.

In a third illustrative non-limiting embodiment, shown in FIG. 4, thecomponent 50 is homogeneous throughout, the sole layer 54 comprisingsome mixture of ceramic and metal. This mixture can be anywhere from 1%to 25% ceramic and anywhere from 99% to 75% metal. Preferably, the levelis 1% to 20% ceramic and 99% to 80% metal. In this embodiment, thelayers comprising ceramic/metal mixtures are characterized by an averagefree distance between ceramic particles that is less than half theaverage size of said ceramic particles.

Although the depicted embodiments illustrate the use of the ceramicmetal composite in use in a hip implant, the ceramic metal compositecould equally be implemented in reconstructive knee components andparticularly in femoral knee components that articulate againstpolyethylene or other types of polymers. The invention described hereinis applicable to orthopedic implants generally, and although thepreferred embodiments are hip implants and knee implants, it is alsoapplicable in, for example, shoulder, vertebral, and other orthopedicimplants familiar to those of skill in the art.

As various modifications could be made to the exemplary embodiments, asdescribed above with reference to the corresponding illustrations,without departing from the scope of the invention, it is intended thatall matter contained in the foregoing description and shown in theaccompanying drawings shall be interpreted as illustrative rather thanlimiting. Thus, the breadth and scope of the present invention shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims appendedhereto and their equivalents.

1. An orthopedic implant component comprising: a homogenous composition comprising a metallic species and a secondary phase wherein said secondary phase comprises a ceramic, and wherein the average free distance between ceramic particles is less than the average size of said ceramic particles; or, a multilayered composition wherein one or more layers comprise a ceramic, a metallic species, or a combination of a ceramic and a metallic species.
 2. The implant component of claim 1, wherein said ceramic is selected from the group consisting of alumina, zirconia, chromium carbide, chromium nitride, silicon carbide, silicon nitride, titanium carbide, zirconium carbide, zirconium nitride, tantalum carbide, tungsten carbide, and any combination thereof.
 3. The implant component of claim 1, wherein said metallic species is selected from the alloy groups consisting of cobalt-chromium, titanium- aluminum-vanadium, zirconium-niobium and tantalum and any combination thereof.
 4. The implant component of claim 1, wherein said multilayered composition has a surface layer of homogeneous ceramic, and a subsurface layer comprising a metallic species and a secondary phase comprising a ceramic immediately below said surface layer and forming a boundary with said surface layer.
 5. The implant component of claim 4, wherein said surface layer of homogeneous ceramic has a thickness of about 25 μm.
 6. The implant component of claim 4, wherein said surface layer is 100% alumina.
 7. The implant component of claim 4, wherein said surface layer is 80% alumina by volume and 20% zirconia by volume.
 8. The implant component of claim 4, wherein said subsurface layer comprising a metallic species and a secondary phase comprising a ceramic is a graded layer wherein the level of ceramic decreases from an initial concentration at the boundary with said surface layer to depths further from said boundary with said surface layer.
 9. The implant component of claim 8, further comprising a homogenous metallic substrate layer immediately below said graded layer.
 10. The implant component of claim 9, wherein said metallic substrate layer is a homogeneous cobalt-chromium alloy.
 11. The implant component of claim 4, wherein said concentration at said boundary of said ceramic in said subsurface layer is 98% by volume.
 12. The implant component of claim 1, comprising a multilayered composition. wherein one layer comprises a combination of a ceramic and a metallic species and forms a surface of said implant component and wherein said one layer is a graded layer wherein the level of ceramic decreases from an initial concentration at said surface to depths further from said surface.
 13. The implant component of claim 12, wherein said concentration at said surface of said ceramic is 98% by volume.
 14. The implant component of claim 1, comprising a homogenous composition comprising a metallic species and a secondary phase wherein said secondary phase comprises a ceramic and wherein said ceramic comprises alumina.
 15. The implant component of claim 14, wherein said ceramic is a mixture of alumina and zirconia.
 16. The implant component of claim 1, wherein said implant component comprises a hip implant.
 17. The implant component of claim 1, wherein said implant component comprises a knee implant.
 18. The implant component of claim 1, comprising a homogenous composition comprising a metallic species and a secondary phase wherein said secondary phase comprises a ceramic, and wherein the average free distance between ceramic particles is less than half of the average size of said ceramic particles.
 19. The implant component of claim 1, comprising a multilayered composition wherein one or more layers comprise a combination of a ceramic and a metallic species and wherein the average free distance between ceramic particles is less than the average size of said ceramic particles.
 20. The implant component of claim 19, comprising a multilayered composition wherein one or more layers comprise a combination of a ceramic and a metallic species and wherein the average free distance between ceramic particles is less than half of the average size of said ceramic particles.
 21. A method of making an orthopedic implant component having a graded composition, comprising the steps of: mixing powders of ceramic components and metal components to desired ratios to form a first mixture; mixing powders of ceramic components and metal components to desired ratios to form a second mixture; laying said first mixture on said second mixture to create a first layered mixture; optionally mixing powders to form one or more additional mixtures and laying said one or more additional mixtures on said first layered mixture; and, sintering said first layered mixture or sintering a combination of said first layered mixture and said one or more additional mixtures.
 22. The method of claim 21, wherein at least one mixing step comprises mixing using a ball milling process.
 23. The method of claim 21, wherein at least one mixing step comprises conventional high temperature sintering.
 24. The method of claim 21, wherein at least one mixing step comprises electric field assisted sintering.
 25. The method of claim 21, wherein at least one mixing step comprises spark plasma sintering.
 26. The method of claim 21, wherein said metal components have a Young's modulus in the range of about 60 GPa to about 150 GPa.
 27. The method of claim 21, further comprising the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles.
 28. The method of claim 27, wherein said step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles comprises altering the size and volume fraction such that the average free distance between ceramic particles is less than half of the size of said ceramic particles. 